1. Field of the Invention
The present invention relates to a method for producing a toner and a toner produced by the method which is excellent in productivity and economic efficiency, in which in a milling and classifying step of the toner, pulverized particles contained more than required in the toner as a product are accurately classified, and the toner having excellent quality property can be stably and easily produced.
2. Description of the Related Art
Conventionally, for the method for milling and classifying the toner (1) a pair of a classifier and a mill or two or more pairs thereof, (2) a combination of two classifiers and a mill or the like have been proposed (Japanese Patent (JP-B) No. 2851872, Japanese Patent Application Publication (JP-B) No. 6-66034, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-275685 and 11-15194, and JP-B No. 3748555). For example, a jet mill unit, so called jet mill, in which high pressure airflow is blown out from a jet nozzle to involve raw material particles therein, and then the particles collide each other, or collide to a wall or other impactor. In the jet mill, at first particles are milled by one or two milling units and two classifying units of coarse particles, and then pulverized particles are classified by at least one classifying unit.
FIG. 1 shows an example of a flow diagram of the milling and classifying step of the conventional toner. In the flow shown in FIG. 1, a raw material is supplied from a raw material supply part 1, introduced to a first classifier 2, and then classified into coarse particles and pulverized particles. The pulverized particles are recovered in a first cyclone unit 4, while the coarse particles are milled in a first mill 3 and then once recovered in the first cyclone unit 4. Next, the particles in the first cyclone unit 4 is introduced to a second classifier 6, and then classified into coarse particles and pulverized particles. The pulverized particles are recovered in a second cyclone unit 8, while the coarse particles are milled in the second mill 7, and then recovered in the second cyclone unit 8. The particles in the second cyclone unit 8 are introduced to a third classifier 10 and classified into coarse particles and pulverized particles. The coarse particles are recovered as a toner product 11, while the pulverized particles are once recovered in a third cyclone unit 12, and then further classified into coarse particles and pulverized particles in the fourth classifier 13. The pulverized particles are recovered in a fourth cyclone unit 14, while the coarse particles are returned to the third classifier 10 through a return pipe 13a, and the classification is repeated until the desired particle size is obtained. The pulverized particles are collected in the fourth cyclone unit 14 as a pulverized particles 16. Moreover, the pulverized particles are collected from the upper part of the third cyclone unit 12 and the fourth cyclone unit 14 as well as the upper part of the third classifier 10 and the fourth classifier 13 by the third collector 15. The collected pulverized particles are granulated and used or directly used again as a kneading product.
In the flow of the milling and classifying step shown in FIG. 1, the coarse particles classified in the fourth classifier 13 are returned to the third classifier 10, thus a burden to the third classifier 10 is increased. Moreover, because the amount of the particles returned from the fourth classifier 13 is not constant, the classified density of the third classifier 10 fluctuates, the stable particle diameter distribution cannot be obtained and the accuracy of classification may be decreased. When the toner obtained by the above-mentioned flow of the milling and classifying step is used to form an image, background smear may occur due to unstable image density and charge amount, and image quality may be decreased due to transfer failure.
To obtain a desired toner particle size, excessive removal of pulverized particles leads to the reduction of a yield of toner product. As a result, the amount of the collected pulverized particles is increased, and force loading for reuse is increased, and economic disadvantages may be invited, for example, worse productive energy efficiency, cost rise, and production of excess CO2.